What term describes a gradual decrease in intensity or volume?

The term that describes a gradual decrease in intensity or volume is "diminuendo." In music, diminuendo indicates that the performer should gradually reduce the loudness of the sound, creating a fading effect. This is commonly used to convey emotion or create a sense of conclusion in a musical passage.

Crescendo, in contrast, refers to a gradual increase in intensity or volume, making it the opposite of diminuendo. Ritard is a term that indicates a gradual slowing down of the tempo rather than a change in volume. A tempo refers to returning to the original speed of the piece after a change, and it does not denote any change in intensity or volume. Therefore, "diminuendo" is the appropriate choice to represent a decrease in loudness in a musical context.